From Sahih Muslim (Hadith 4538):
It is narrated on the authority of Abu Sa'id that the Messenger of Allah (ﷺ) said:On the Day of Judgment there will be a flag for every person guilty of the breach of faith. It will be raised in proportion to the extent of his guilt; and there is no guilt of treachery more serious than the one committed by the ruler of men
Reference: Muslim 4538